1. Field of the Invention
The present invention relates to code-division multiplex communications.
2. Description of the Related Art
FIG. 1 shows the configuration of the spreading unit of a conventional code-division multiplex communications system.
Data series DataI1, DataI2, . . . DataQ1, DataQ2, . . . , (although four data series ranging from data series I1 to data series Q2 are shown in FIG. 1, there is no limitation to the number of data series) are modulated/transmitted after being spread using the respective spreading code, and subsequently are IQ-scrambled using a scrambling code (the I signal and Q signal are scrambled using a scrambling code).
Specifically, data series I1 (DataI1) is spread by multiplying it by a spreading code I1 in a multiplier 900-1. In the same way, data series I2 (DataI2) is spread using a spreading code I2 in a multiplier 900-2. The data series I1 and I2 spread using different spreading codes are added in an adder 901-1, and are inputted to a multiplier 902-1 and a multifier 902-2, respectively.
Data Q1 and Q2 (DataQ1 and DataQ2) are multiplied by spreading codes Q1 and Q2, respectively, in multipliers 900-3 and 900-4, respectively, and are added in an adder 901-2. Then, the output of the adder 901-2 is inputted to multipliers 902-3 and 902-4. Then, in multipliers 902-1 and 902-2, the output signal is multiplied by scrambling codes cI and cQ. In the same way, in multipliers 903-3 and 903-4, the output signal of the adder 901-2 is multiplied by scrambling codes cI and cQ. The outputs of both multipliers 902-1 and 902-3 are inputted to an adder 903-1, and are added. The outputs of the multipliers 902-2 and 902-4 are inputted to an adder 903-2, and are added. Then, the output of the adder 903-1 is inputted to a QPSK modulator 904 as an I signal, and the output of the adder 903-2 is inputted to the QPSK modulator 904 as a Q signal.
A scrambling code is used for identifying each channel, and data I1 through Q2 are transmitted on different channels. Furthermore, a scrambling code is uniquely assigned to a user, and signals spread by the same scrambling code are outputted from one user terminal.
Since data series I1 through Q2 are put on different channels, they are multiplied by different codes I1 through Q2, respectively. However, if these data series are brought together and are transmitted from one user as one originating signal, for example, I1 and Q1, I2 and Q2 are separated into two different sets of data consisting of an I signal and a Q signal transmitted from the user, and simultaneously, scrambling codes of cI and cQ are prepared. The multiplication by a scrambling code of a signal spread by a spreading code is performed by a circuit performing an operation similar to the multiplication of a complex number when both the scrambling code and the spread signal are treated as complex signals.
Specifically, assuming that data series I1 (DI1) and data series Q1 (DQ1), and data series I2 (DI2) and data series Q2 (DQ2) are each part of a pair of complex number data, the circuit is configured so as to perform the following calculation.(DI1SI1+jDQ1SQ1)(cI+jcQ)+(DI2SI2+jDQ2SQ2)(cI+jcQ)=(DI1SI1+DI2SI2)(cI+jcQ)+j(DQ1SQ1+DQ2SQ2)(cI+jcQ)=((DI1SI1+DI2SI2)cI−(DQ1SQ1+DQ2SQ2)cQ)+j(DI1SI130 DI2SI2)cQ−(DQ1SQ1+DQ2SQ2)cI)  (1),wherein j is the imaginary unit, and SI1, SI2, SQ1 and SQ2 are spreading codes for I1, I2, Q1 and Q2, respectively. The data series are modulated by a QPSK modulator 904 treating the real element and imaginary element of a complex number expressed by equation (1) as an I signal and a Q signal, respectively. Since as shown in equation (1), the sign of the second term of the I signal is negative, as shown in FIG. 1, the sign of signals outputted from a multiplier 902-3 are inverted and inputted to an adder 903-1.
Although in FIG. 1 the number of channels used by one user is four, the number of elements is not limited to this number, allowing an arbitrary number of channels can be assigned to one user. In this case, too, a pair of the two data series signals of each channel are handled as a complex number, and the circuit is configured in such a way that the result of multiplication by a scrambling code, which is also handled as a complex number, may be the same as the result of multiplication between complex numbers as in equation (1).
FIG. 2 shows the configuration of a conventional receiver for receiving signals transmitted in the conventional code-division multiplex communications system shown in FIG. 1.
On a receiving side, a received signal is multiplied by the complex conjugate of a scrambling code used on a transmitting side, the multiplied signal is despread using a spreading code, thereby obtaining the original data. In the example shown in FIG. 1, data series DI1 and DQ1 are handled as information series to be transmitted and a pilot symbol for estimating the parameter of a propagation path, respectively. For DQ1, a data series known in advance on the receiving side is used. The reception signal (signal despread using a spreading code SQ1) of a pilot symbol is divided by this DQ1, thereby obtaining the estimation value α of the parameter of a propagation path.
Specifically, a signal received by an antenna 1000 is multiplied by the complex conjugate of a scrambling code used on the transmitting side in a multiplier 1004. To multiply a signal by the complex conjugate of a scrambling code means to multiply a signal regarding two elements of the scrambling code, cI and cQ, as the real and imaginary elements of one complex signal when a received signal is treated as a complex signal consisting of an I signal and a Q signal. Although the details of a circuit for performing the multiplication is not shown in FIG. 2, the fact that the conventional configuration of a digital signal process circuit can be utilized will be easily understandable to a person having an ordinary skill in the art. A symbol of (t−τ) attached to the complex conjugate cI−jcQ of a scrambling code in FIG. 2 indicates that proper synchronous catching and synchronous holding are performed, and the received signal and the complex conjugate of a scrambling code are multiplied while being synchronized. τ indicates a delay in phase. If (t−τ) is attached to the symbol, etc., of a scrambling code or a spreading code like this, it means that synchronization is performed along with the multiplication of a code by way of a proper timing adjustment.
After the received signal is multiplied by the complex conjugate of a scrambling code in the multiplier 1004 and scrambling of it is cancelled, the cancelled signal is branched and is multiplied by a spreading code I1 in a multiplier 1005-1. Thus data I1 are demodulated. In a multiplier 1005-2, the received signal is multiplied by a spreading code Q1, thus data Q1 are demodulated. It is assumed here that data Q1 are pilot signals (pilot symbols). A pilot signal is a signal known on the receiving side which is predetermined in a system. The demodulated pilot signal is inputted to a divider 1007, and is divided by the symbol DQ1 of a predetermined pilot signal prior to transmission. By such a process, a value indicating the degree of fluctuation against its original signal due to fading of a received pilot signal, specifically both the degree of fluctuation of its amplitude and the degree of rotation of its phase, is calculated. The operation result is treated as the estimated value α of a channel estimation, and its complex conjugate is inputted to a multiplier 1006. The demodulated data signal DI1 is also inputted to the multiplier 1006, and the phase rotation caused in a transmission line is compensated for by multiplying DI1 by the complex conjugate of the estimation value α. In this way, phase rotation is compensated for, and simultaneously a data signal DI1 with an amplitude value that fluctuates according to the amplitude fluctuation ratio of the pilot signal is inputted to a RAKE combiner 10002, and is combined with the signal of another path inputted from a despreading unit 1001-2 having the same configuration as a despreading unit 1001-1. The RAKE-combined signal is inputted to a signal decision unit 1003, and its signal value is determined in such a form where a noise, etc., is eliminated.
Although for RAKE-reception, only two of the despreading units for multi-path reception are described in FIG. 2, there is also a configuration such that more despreading units are provided, thereby allowing more multi-paths to be received.
FIG. 3 shows the amplitude-phase diagram (constellation) of a signal transmitted in the conventional code-division multiplex communications system shown in FIG. 1. In this example, data series DataI1, DataI2, DataQ1 and DataQ2 have the same amplitude of ±1. Since in this case, a transmission signal is obtained by multiplexing the four data series, the signal has an amplitude of 2√{square root over ( )}2 at one instant, an amplitude of 2 at another instant and no amplitude at another instant. If a signal with a large maximum-to-minimum ratio or a signal with a large maximum amplitude compared with average power, is amplified by an amplifier, a high linearity is required for the input/output characteristic of an amplifier. Generally speaking, an amplifier with a highly linear input/output characteristic has low efficiency, a signalling system in which the fluctuation of an amplitude is small and a ratio of the maximum amplitude to average power is small, is desirable from the viewpoints of both power efficiency and heat generation.